Field of the Invention
This invention relates generally to the field of ultrasonic imaging techniques in which ultrasonic energy is utilized to produce a visual image of internal structure of a subject. More particularly, the invention relates to improved apparatus and method for synthesizing precisely controlled programs of clock signal frequencies used to operate certain delay circuitry in imaging electrical circuitry of such ultrasonic imaging systems.
Typically, such ultrasonic systems include a multi-element transducer, a display, and intervening imaging electronic circuitry. The transducer produced ultrasonic energy which is directed into the subject, and produces electrical signals representing characteristics of ultrasonic echoes produced by the incident energy at interfaces between tissues of differing nature within the subject. The imaging electronics processed the electrical echo-representing signals from the transducer so that they can be employed to actuate the display (such as a cathode ray oscilloscope) to produce a visual representation of structure within the subject.
A known mode of operation of a system such as the one described above is called "B scanning". In B scanning, the transducer is actuated by the imaging electronics to direct pulses of ultrasonic energy into the subject along a beam path. Synchronously with the production of each pulse, the imaging electronics causes the display oscilloscope to initiate a trace in the form of an electron beam and to move the electron beam in a trace across the oscilloscope screen in a direction which is a function of the direction of the ultrasonic beam path.
When an ultrasonic pulse of energy traverses an interface within the subject between tissues of differing acoustical characteristics, an ultrasonic echo is produced, which is utilized to modulate the intensity of the electron beam trace to derive a visual indication on the display or beam trace to derive a visual indication on the display of the location and characteristic of the interface within the subject as expressed by the echo intensity.
This occurs by virtue of the fact that some of the energy from the ultrasonic echoes is directed back toward, and is received by, the transducer. In response to the receipt of the ultrasonic echos, the transducer produces electrical signals which are processed by the imaging electronics and thereby employed to modulate the intensity of the electron beam trace of the display to produce the indication of the location and characteristic of the interface.
An operator using a B scanning apparatus, by moving the transducer about the periphery of a subject in a common place through the subject, can "paint" an image on the oscilloscope of internal subject structure in the plane.
It is known to employ the multiple elements of the transducer to the receipt of ultrasonic echoes in a particular region. A proposal for doing this involves the use of a transducer having several concentric annular transducer elements. The transducer elements are separately connected to the imaging electronics so that they can be independently actuated to produce the incident ultrasonic beam. Similarly, the individual transducer elements are electrically segregated so that electrical signals they produce from the ultrasonic echoes can be separately processed.
The focusing of the transducer sensitivity in a predetermined region is accomplished by appropriately delaying the signals from the various transducer elements relative to one another. This delay is accomplished by the use of separate delay circuit elements interposed in each of the various individual electrical circuit connected respectively to each of the transducer elements.
In order to focus the sensitivity of the transducer at a particular distance from the transducer, the delay circuits connected to the respective transducer elements are operated to establish a predetermined pattern of delay among the circuits. This delay pattern is selected such that the electrical signals produced by each transducer element in response to echoes emanating from within the predetermined region are placed in phase with one another. This co-phasing of the electrical signals produced by the transducer enables these signals to reinforce one another, rather than to interfere with one another as would be the case if the signals were not appropriately phase delayed. This reinforcement of the various electrical signals tends to intensify their combined amplitude, and renders the ultrasonic system generally more sensitive to ultrasonic echoes emanating from within the predetermined region than would be the case without the appropriate delay pattern.
It is also known to provide means for time varying the distance from the transducer of the predetermined region of heightened sensitivity. If the predetermined region of optimum sensitivity is caused to recede outwardly from the transducer, beginning with the initiation of a particular ultrasonic energy pulse, and the speed of movement of the region equals the acoustic velocity in the subject, reception of the echoes is enhanced. The enhancement of reception occurs because the predetermined region of optimum sensitivity is caused to coincidentally recede into the subject with the incident wavefront in response to which ultrasonic echoes are created.
Specific techniques for deriving concrete delay patterns such as described above are explained by Walker, J. T. et al in "Digitally Controlled CCD Dynamically Focused Phased Array" 1975 Ultrasonics Symposium Proceedings, I.E.E.E. Cat. #75 CHO 994-45U, expressly incorporated here by reference.
For the previously described "fixed focus" system, where the region of enhanced sensitivity is located in one particular place, fixed delay lines are used. These fixed delay lines are hard wired circuitry including series connected branches having capacitive and resistive circuit elements. In such delay lines, an electrical pulse appearing at one end of the series of delay line branches requires a predetermined time to pass to an output connected to the last of the series connected elements. An advantage of these fixed delay lines is their relative economy and simplicity. In fixed focus systems, where the delay in the transmission of electrical signals from each of the transducer elements can be constant, and the delay need not vary, these fixed delay lines are acceptable means for accomplishing the delay.
Where, however, it is desired to time vary the focal distance of the transducer's region of sensitivity, and time variable delays must be applied to the various electrical signals emanating from the respective transducers, other means must be used to effect these delays.
It is known to use delay circuitry in such instances in which input pulses pass through the delay circuitry element in a time which is a function of the frequency of a clocking signal which is applied to that delay circuitry. Accordingly, in order to vary the delay time of a particular delay element, it must be possible to alter the frequency of the clocking signal applied to that element.
It is obvious that in an application such as the ultrasonic system described herein, the delay times of the various delay lines employed must be made to vary with extreme rapidity in order to adjust the delay rapidly enough to accomplish the desired change of the focal distance. In practice, it has been determined that it is often necessary to employ use times for the various clocking frequencies applied to the delay elements of as little as 100 microseconds or less.
Existing technology has not yielded apparatus which can accomplish the very rapid frequency changes in the clocking signals with the precision required.
One proposal for controlling these frequencies has been to employ circuitry known as a phase locked loop. Such circuitry typically employs a constant frequency source along with a voltage controlled oscillator. A voltage controlled oscillator is a device which produces an alternating output signal whose frequency is a function of an analog voltage level input to the voltage controlled oscillator. A phase locked loop also employs a phase detector which produces an analog output whose magnitude is a function of the difference in phase between two input signals. The output of the phase detector is connected to the input of the voltage controlled oscillator and its inputs are the output from the constant frequency source and the output from a variable divider circuit. The variable divider circuit produces an alternating signal having a frequency which is a submultiple of the frequency of the output which is applied to the divider. In a phase locked loop, the input to the variable divider is derived from the output of the voltage controlled oscillator and the output of the variable divided is directed as one input to the phase detector.
A phase locked loop such as described here can be used to generate an output signal having a frequency extending between a range of F1 to F2 in N discrete steps separated by a frequency difference D. When so employed, the constant frequency source is selected to produce a signal having a frequency D. The variable divider circuit is adjustable in steps to produce a signal having a frequency of F down to F/N where the variable divider is set to dive by integers from 1 to N.
It has been determined, however, that a phase locked loop is generally not useful for controlling frequencies in the environment of an ultrasonic system such as described here. This is because the phase locked loop lacks the required circuit stability to accomplish precise frequency control of its output with the rapidity dictated by the very short use times appropriate in employment in these ultrasonic systems. In present technology, the time for a phase locked loop to lock onto and stabilize at a particular frequency output exceeds considerably the stringent requirement of use times of 100 microseconds or less. It is simply not possible to synthesize a series of programmed frequencies with use times as short as those desired in this application by the use of a phase locked loop. In fact, it is not known that any type of previously existing circuitry could synthesize such frequencies precisely with the required brevity of use time.